Shear Induced Melt Bands: The Mechanics of their Formation and their Possible Role as Melt Conduits Beneath Mid-Ocean Ridges

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• Samuel Butler (University of Saskatchewan)
• Thursday 09 June 2016, 11:30-12:30
• Seminar Room 1, Newton Institute.

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MIMW03 - From the continuum to the tectonic: the magma/mantle dynamics of planet earth

The compaction equations predict that an instability will occur if the matrix viscosity decreases with porosity and the system is subjected to shear. The instability is manifest as a series of bands of high and low porosity. Porosity bands have been seen in experimental investigations of sheared partial melt systems and the bands are always oriented roughly 25° from the direction of maximum compression and occur on length scales similar to the compaction length and significantly larger than the grain size. Linear instability analysis of the compaction equations predicts that bands should grow fastest at the smallest possible length scale and, for purely porosity-dependent matrix viscosity, parallel to the direction of maximum compression. Various additions to the matrix rheology law have successfully been used to produce bands with orientations similar to those seen in experiments, including strain-rate dependent viscosity, anisotropic viscosity and grain-size and roughness dependent or damage rheology. Furthermore, melt bands have been proposed as high permeability conduits that could channel melt towards the mid-ocean ridge.  In order to be effective channels, the bands must be oriented towards the ridge and their amplitude must be sufficient to result in a significant permeability variation after evolving through a mid-ocean ridge corner flow. In this presentation, I will first present linearized theory and numerical modeling results for melt band formation in 2D in simple and pure shear geometries with the rheology laws listed above in order elucidate the process of melt band formation. I will then present an explanation for the growth of the width of melt bands to sizes greater than that of the initial heterogeneity. I will then present linear theory and numerical modeling results for bands formed when the background velocity field is that of a mid-ocean ridge corner flow in order to assess the efficacy of melt bands as a channeling mechanism for melt to mid-ocean ridges. I will show that the rotation of bands by the mid-ocean ridge flow field causes bands to be poorly oriented to channel melt to the mid-ocean ridge and that the amplitude of the bands is only likely to be sufficient to cause the significant permeability heterogeneity that is necessary to cause channelization if the matrix bulk viscosity is small.

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